E. Schumann et M. Ruhle, MICROSTRUCTURAL OBSERVATIONS ON THE OXIDATION OF GAMMA-NI3AL AT HIGH OXYGEN PARTIAL-PRESSURE, Acta metallurgica et materialia, 42(4), 1994, pp. 1481-1487
The oxidation behaviour of gamma'-Ni3Al in air has been studied by tra
nsmission electron microscopy (TEM). Gamma'-Ni3Al single crystals (sur
faces parallel to (001)) were oxidized and cross-sections were prepare
d for TEM examination. After 1 min oxidation simultaneous formation of
an external NiO scale and internal oxidation of gamma'-Ni3Al were obs
erved. The internal oxide particles were identified as gamma-Al2O3, wh
ich possesses a cube-on-cube orientation relationship with respect to
the Ni matrix. After 6 min oxidation a continuous gamma-Al2O3 scale ha
s formed between the internal oxidation zone (IOZ) and the gamma'-Ni3A
l single crystal. Oxidation for 30 min resulted in a microstructure si
milar to the 6 min oxidation, but the Ni in the two-phase zone has bee
n oxidized to NiO. Oxidation for 50 h resulted in a scale consisting o
f an outer layer of NiO, an intermediate layer of NiAl2O4 and an inner
layer of Al2O3. The spinel must have formed by a solid state reaction
between NiO and gamma-Al2O3. The gamma-Al2O3 in the inner Al2O3 layer
transforms to the thermodynamically stable alpha-Al2O3. A crystallogr
aphic orientation relationship exists between both modifications of Al
2O3, whereby (0001)[1100BAR]alpha-Al2O3 parallel-to (111)[110BAR]gamma
-Al2O3, i.e. close packed planes and close packed directions of alpha-
Al2O3 are parallel to the close packed planes and close packed directi
ons in gamma-Al2O3.